Field of the Invention
The present invention relates to a multi-layered porous film, in particular, to a multi-layered porous film suitable to a battery, in particular, suitable to a separator to be used for a nonaqueous-electrolyte secondary battery.
Background Art
Since a nonaqueous-electrolyte secondary battery such as a lithium secondary battery has a high energy density, a nonaqueous-electrolyte battery is broadly employed as a battery to be used in a personal computer, a cellular phone, and a mobile data terminal.
A nonaqueous-electrolyte secondary battery usually has a structure including a winding core, and an outer case in which the winding core is housed, the winding core being comprised of wound four layers including an anode sheet, a separator, a cathode sheet, and a separator. Since the outer case is designed to have a fixed size, if a separator is thick, the battery components such as an anode, a cathode and a separator are housed in the outer case in a much compressed condition. Each of the battery components has a lot of micro pores through which electrolyte passes. If the battery components are much compressed, the micro pores of the battery components (in particular, the micro pores of the separator) are collapsed with the result that an amount of electrolyte to be absorbed into the micro pores is reduced, and accordingly, an adequate amount of electrolyte cannot be supplied to each of the battery components. Furthermore, it takes time for electrolyte after being introduced into a case until penetrating the battery components in the fabrication of a battery, resulting in reduction in productivity of a nonaqueous-electrolyte secondary battery. On the other hand, if a separator is thin, adhesion among an anode, a cathode and a separator is reduced, resulting in that a contact resistance between the battery components is increased, and that electrolyte is in short, because a space is generated in a battery case more than expected.
Furthermore, since a nonaqueous-electrolyte secondary battery has a high energy density, if a nonaqueous-electrolyte secondary battery is damaged or if a device housing a nonaqueous-electrolyte secondary battery therein is damaged to thereby cause internal short-circuit or external short-circuit, a large amount of current runs through a nonaqueous-electrolyte secondary battery, resulting with much generation of heat therein. Accordingly, a nonaqueous-electrolyte secondary battery is required to prevent an amount of heat greater than a predetermined amount from being generated, and further, to keep high security.
In order to keep security, a nonaqueous-electrolyte secondary battery is generally designed to have shut-down function in which passage of ions between an anode and a cathode is interrupted by means of a separator in the case of abnormal heat generation, to thereby avoid further heat generation. As a separator is used a porous film mainly composed of polyolefin fusible in the case of abnormal heat generation. Since the porous film is fused and pores of the porous film are clogged in a battery employing the separator when abnormal heat generation occurs, ions are interrupted to pass through the pores of the porous film to thereby prevent further heat generation. However, in some cases, a separator comprised of a porous film composed of polyolefin may shrink or a porous film may be broken, and accordingly, an anode and a cathode may make direct contact with each other to thereby cause short-circuit. A separator comprised of a porous film composed of polyolefin cannot adequately keep a shape, and sometimes is not able to suppress abnormal heat generation caused by short-circuit.
There may be formed a layer (hereinafter, referred to as “a functional layer”) providing an additional function to the above-mentioned porous film (hereinafter, referred to as “a base film”). For instance, a thermally resistant layer composed of a thermo-resistant material may be formed as a functional layer on a surface of the base film to thereby apply shape-stability to a separator such that the separator keep its shape even in a high temperature environment. As a multi-layered porous film including such a functional layer as mentioned above, there has been suggested a multi-layered porous film on which a functional layer is formed, to be fabricated by coating a coating liquid containing an inorganic filler and a binder resin onto a surface of a base film to thereby form a coating film, and removing solvent from the coating film (see the patent documents 1 to 3).
Apart from the above-mentioned thermally resistant layer, a functional layer may be comprised of an adhesive layer or an electrolyte-retaining layer. A composition and/or a thickness of a functional layer are determined in dependence on a required function.
The above-mentioned functional layer in the multi-layered porous film is fabricated by coating a coating liquid containing both an inorganic filler and a binder resin onto a surface of a base film. A preferable example of a process of coating a coating liquid onto a base film is a gravure coating process. A gravure coating process is particularly suitable to industrial fabrication of a large-sized separator.
A gravure coating process generally includes steps of soaking a gravure roll having protrusions and recesses formed on a surface thereof, into a coating liquid to thereby retain the coating liquid in the recesses formed on a surface of the gravure roll, and causing the gravure roll to make contact with a continuously fed base film to thereby uniformly coat the coating liquid retained in the recesses, onto a surface of the base film.
A gravure roll to be used for a gravure coating process is designed to have various patterns of protrusions and recesses to be formed on a surface thereof, and a depth of recesses in accordance with a desired coating pattern and/or a coating volume for various purposes.